The present invention relates, in general, to kilns for drying and, in particular, to a new and useful control system for temperature and humidity within a kiln for drying hardwoods.
It is known that by controlling the temperature and humidity within the interior of a hardwood kiln, lumber positioned within the kiln may be dried at a controlled rate to minimize drying related defects of the lumber as it is dried. In order to control the kiln temperature and humidity, a sample piece of lumber is selected from the lumber being dried, and the moisture content of the sample is determined outside the kiln, at preselected intervals of time. With the moisture content information, the kiln temperature and humidity are altered, if necessary, to ensure that the lumber is dried at the desired rate.
The moisture content of the lumber sample is typically determined by periodically removing the sample from the kiln and weighing the removed sample or measuring an electrical characteristic, such as the conductivity, of the removed sample. The moisture content of the lumber sample may then be calculated or otherwise derived from the information obtained from the removed sample.
Such a process is time-consuming, is susceptible to error in calculating the moisture content, is likely to effect the environment of the kiln interior as the kiln interior is exposed to ambient conditions during removal of the sample from the kiln and is uncomfortable and potentially dangerous in that the operator must enter the hot kiln. In addition, the sample used to monitor the moisture content is re-positioned within the bulk of the lumber situated within the kiln following each moisture-determining operation. Such a re-positioning of the sample renders it inaccessible for unhindered visual inspection.
The drying of hardwood lumber requires kilns of significantly different design from those for drying softwood lumber. Hardwood drying kilns must be controlled as to temperature and humidity and to a far more accurate extent. For example, kilns for drying softwood such as pine, can use simple thermocouples for monitoring temperature. Much more accurate temperature sensors are needed in kilns for drying hardwoods, however, since control of drying to 1.degree. Fahrenheit is necessary and critical. For additional details, see the article "Process Control in Drying Hardwood Lumber", R. L. Little, Sensors, September 1988, pages 49-57.
In hardwood kilns, the conventional wisdom requires that multiple samples which are removed from the kiln for periodic moisture measurements, be repositioned within the lumber charge in a horizontal orientation, that is parallel to the air stream through the charge. As will be explained in the following, the present invention represents a substantial diversion from conventional thinking and results in a highly accurate measurement apparatus and technique for controlling hardwood kilns.
U.S. Pat. No. 1,593,890 to Welch, discloses an apparatus for drying wood which includes a balance beam for weighing a wood sample held in a horizontally extending position and at a non-disclosed location in the kiln. Nothing is mentioned in the reference on the survivability of the equipment within the corrosive, quickly-moving, hot, moist atmosphere in a kiln.